A known throttle valve control device is disclosed, for example, in Japanese Laid-Open Publication No. Hei 07(1995)-97950. The throttle valve control device includes a throttle valve, a gear mechanism, a DC motor, an electronic control unit (ECU), a throttle valve position sensor and an accelerator pedal sensor. The throttle valve position sensor detects the actual throttle valve position and outputs a throttle valve position signal to the ECU. The accelerator pedal sensor detects the actual accelerator pedal position and outputs an accelerator pedal position signal to the ECU. The ECU determines a target throttle valve position in response to the actual accelerator pedal position and other parameters representing engine driving conditions, for example, the amount of fuel injection to the engine and the temperature of the engine. The gear mechanism is disposed between the DC motor and the throttle valve to transmit the rotating torque from the DC motor to the throttle valve. The DC motor is turned on electrically by the ECU to drive the throttle valve via the gear mechanism. That is, the throttle valve is opened and closed by the DC motor which is controlled by the ECU. The ECU performs a servo-control based on Proportional Integral Derivative control (PID control) such that the actual throttle valve position corresponds to the target throttle valve position.
Generally speaking, for purposes of rotating the throttle valve within a predetermined range, the throttle valve control device has two stoppers. One stopper is a full opening stopper which is able to contact a part of the throttle valve when the throttle valve is positioned at the maximum opening position in the predetermined range. The other stopper is a closing stopper which is able to contact another part of the throttle valve when the throttle valve is positioned at the complete closing position or minimum opening position in the predetermined range. Therefore, if the throttle valve control device is in an abnormal state, for example when the throttle valve receives an excessive rotational torque, the position of the throttle valve is maintained in the predetermined range.
However, when the throttle valve control device is in the abnormal state by virtue of changing conditions, for example a change in environmental temperature or a change in voltage of the power source, the stoppers receive excessive torque. Accordingly, the DC motor and the parts of the gear mechanism are susceptible to becoming broken.
In an attempt to address this problem, it is of course possible to increase the strength of the parts. However, this increases the weight and the moment of inertia of the parts, thus decreasing the operating response.
A need thus exists for a throttle value control device that is not excessively heavy and does not have an excessively large moment of inertia, but which nevertheless is not susceptible to damage and breakage of the DC motor and gear parts.